By William Oh, Electro Static Technology
Encouraging results from recent up-tower testing at a wind farm in Oregon indicate that the conductive-microfiber bearing protection ring is preventing generator bearing damage. Designed by the Electro Static Technology engineering team to protect wind turbine motor bearings, the ring appears to have solved a chronic bearing failure problem.
The generator bearings at the top of one turbine tower first failed in May 2006, 11 months after the tower was brought online. The company that owns and operates the wind farm replaced the bearings and slip rings, but the new bearings failed five months later. Once again, new bearings and slip rings were installed.
The third bearing failure came 11 months later in September 2007. This time, in addition to replacing another set of insulated (ceramic-coated) bearings and slip rings on both ends of the generator, the owner decided to install Electro Static Technology’s conductive-microfiber bearing protection ring and the shaft collar with its highly conductive surface on the drive end. The generator’s two standard carbon block, spring-loaded brushes, which rub on the slip ring at the non-drive end, were also replaced.
Three months later, a probe and oscilloscope measured the shaft voltage on the generator with and without the Electro Static Technology bearing protection ring and collar engaged. All measurements were taken on the same circuit (Figure 1). Wind speed ranged from 10.2 to 13.4 mph. Real-time data from these field tests show that the conductive-microfiber bearing protection ring and collar were reducing shaft voltage by an average of 84.5 percent.
The first measurement, taken during full-power operation with a wind speed of 12.1 mph, established a baseline voltage (the system’s ground noise level) of 2.60 volts (peak-to-peak) from the 5.824” shaft of the tower’s doubly fed, asynchronous generator. The oscilloscope setting was 10.0 V/div, 400 ms/div.
The crew then conducted eight more measurements in two series. The Series 1 readings measured the shaft voltage with all components engaged. The bearing protection ring and collar were on the drive end of the shaft, and the standard carbon block brushes were on the non-drive end. For the Series 2 readings, the bearing protection ring was disengaged and the shaft collar was removed, leaving the non-drive-end carbon block brushes as the only shaft-current mitigation.
The growth of wind power highlights a range of maintenance issues.
High-frequency currents induced on the shafts of wind turbine generators through parasitic capacitive coupling can reach levels of 60 amps and 1,200 volts or greater. If not diverted, these currents discharge through the generator’s bearings, causing pitting and fluting that result in premature bearing failure and catastrophic turbine failure. The Electro Static Technology ring technology steers these currents away from the bearings and channels them to ground.
The ring surrounds the generator shaft with millions of conductive microfibers of less than 10 microns in diameter. These fibers provide a high density of contact points, parallel paths of least resistance from the motor shaft to ground. Capable of conducting currents of many tens of amperes and discharging from tens to thousands of volts with frequencies in the MHz range, the fibers reduce voltage build-up on the generator shaft. The ring is suitable for use at high frequencies because its fibers tend to compensate for variations in the roughness of the shaft surface and/or microscopic misalignment of the ring and shaft.
When the microfibers lose mechanical contact with the rotating shaft, electric contact is re-established somewhere along the ring, due to local field emission. When the gap between the shaft and the fibers is relatively large (greater than 5 microns), this is accomplished through the phenomenon known as a gaseous or electric “breakdown,” a cascading effect of secondary electrons obtained by collisions and impact ionization of the gas ions accelerating across the gap. With a smaller gap (5 nanometers to 5 microns), field emission is a form of quantum tunneling known as Fowler-Nordheim tunneling, a process in which electrons “tunnel” through a barrier in the presence of an electric field.
Thus, the ring fulfills all the functions of conventional spring-loaded carbon brushes with neither the direct frictional wear nor the hot-spotting/thermal wear common to such brushes. And because multiple microfibers dissipate heat better than single-conductor devices, the ring can tolerate higher current densities. Furthermore, the microfibers are not adversely affected by oil, grease, dust, moisture or other contaminants.
More specifically, the wind turbine bearing protection ring is engineered to divert up to 120 amps of continuous high frequency shaft current at frequencies as high as 13.5 MHz and discharges of up to 3,000 volts (peak). The ring is suitable for up-tower retrofits and preventive maintenance programs as well as for original equipment manufacturer installation.
Results of the Series 1 tests of the turbine with the bearing protection ring installed show an average shaft voltage of 6.41 volts (peak-to-peak). Results of the Series 2 tests where only the spring-loaded carbon block brushes were used show an average shaft voltage of 41.35 volts (peak-to-peak). The difference between these figures, 34.94 volts, indicates that the bearing protection ring and collar successfully divert approximately 84.5 percent of the damaging current that remains on the tower’s generator shaft when the only bearing protection is from the carbon block brushes at the non-drive end. The voltage wave form with our ring and collar engaged was a smooth wave with no detectable discharge to the bearings, while the wave form without our ring and collar showed a bearing-current-discharge pattern with voltage peaks an average of 6.5 times higher.
These measurements show that the bearing protection ring lowers shaft voltages and thus mitigates the destructive impacts of shaft current discharges to bearings in wind turbine generators. Damaged bearings can cause generator failures, which leads to repairs and unplanned downtime. The return on investment for preventing such failure can be quite high.
Author: William Oh is general manager of Electro Static Technology, an Illinois Tool Works company, and a leader in the development and application of passive ionization technology. With degrees in mechanical engineering from Pusan National University and the Korean Advanced Institute of Science and Technology, he is a member of the Illinois Tool Works Patent Society.